This application is based on PCT/EP99/01726 filed on Mar. 17, 1999, which claims priority from German No. 198 11 736.1 filed on Mar. 18, 1998.
1. Field of the Invention
The invention relates to a method for varying the swirling movement of a fluid in the swirl chamber of a nozzle, and to a nozzle system for carrying out the method. Such nozzles are used, in particular, in industrial burners, oil burners and systems for washing flue gas and for the spray-drying of foodstuffs.
2. The Prior Art
It is frequently desired to be able to vary the atomization characteristic when atomizing liquids with the aid of swirl nozzles. It is possible to influence the drop size of the spray produced by varying the circumferential speed (swirling movement or swirl component) of the fluid in the swirl chamber. It is important here that the circumferential speed can be varied independently of the liquid throughput, and also that there is no need to undertake mechanical variation at the nozzle. So-called spill-return nozzles (bypass nozzles) constitute a variant. With these nozzles, the liquid is directed tangentially into the swirl chamber and drained off both from the nozzle outlet opening and through a return-flow opening on the middle of the axis. This portion of the liquid throughput is led back again into the liquid reservoir. By varying the return rate, the liquid throughput which is atomized can be kept constant, although the inlet speed of the liquid into the swirl chamber can be varied and thereby adjusted to the swirl intensity and, consequently, to the drop quality. The disadvantage of this solution consists in the necessity of conducting liquid in a circuit. The control range of the spill-return nozzles is bounded below. There is a substantial variation in the jet angle with the desired control range.
Also known are so-called xe2x80x9cduplex nozzlesxe2x80x9d (DE-C 893 133 and U.S. Pat. No. 2,628,867), which are used for atomizing fuels. The nozzles have a swirl chamber into which the fuel is introduced via a plurality of tangential feed channels, and is set rotating about an axis. The nozzles can have different cross-sectional surfaces at the connecting point to the swirl chamber, and the tangential feed channels are connected to separate feed conduits. Incorporated into one of the feed conduits inside the nozzle is a valve which is opened as a function of the pilot pressure present in the other feed conduit, and permits the feed of a larger fuel quantity. The disadvantage of the xe2x80x9cduplex nozzlesxe2x80x9d resides chiefly in the fact that they can implement only a limited possibility of regulation and control which is a function of the pilot pressure present or throughput. U.S. Pat. No. 4,796,815 describes a shower head for a hand-held shower in the case of which the incoming water flow is introduced via two tangential and two radial channels into a swirl chamber, in which a rotatable ball is also located, as well. The water feed in the nozzle head may be varied by means of an adjusting element which can be actuated by hand; either the water inlet into the tangential channels or into the radial channels is covered, or the radial and tangential channels are only partially covered. Different spray patterns are obtained by means of these possible adjustments. The disadvantage of this spray head consists in that for the purpose of generating different spray patterns the adjusting element is arranged inside the swirl chamber, and this varies the inlet surfaces of the tangential and radial channels. This shower head is essentially limited in its application to the sanitary field.
DE 39 36 080 C2 has disclosed a method for varying the circumferential speed component of the swirl flow of a fluid at the outlet from a swirl nozzle having a swirl space with a plurality of tangential feed lines. The entire material flow of the fluid is subdivided into at least two subflows, it being possible to vary the size of at least one subflow. The subflows are fed into the tangential feed conduits of the swirl space. It is disadvantageous that the achievable control range depends on the number of the feed conduits, the result being a rise in the outlay of production for the nozzles with a wide control range. Although rotational symmetry of the flow is achieved, the control range remains narrow. The known nozzles for industrial burners have the disadvantage that the burner output must be kept constant, because otherwise undesired pollutant emissions occur, in particular when the throughput is varied. Remedy is frequently found with a plurality of nozzles, it being possible to achieve optimum conditions only for one operating case. With the known nozzle systems used in spray-drying, a system start-up time of 2 to 3 hours is required when switching products. The powder produced during the start-up time cannot be reused, and must be recycled with considerable outlay. Moreover, it is not possible to influence variations in the product quality and product specification during the operation of production with the aid of the known nozzle systems. The reason for these disadvantages in the known swirl nozzles is their limited and/or inadequate control range.
It was the object of the invention to create an improved method for varying the swirling movement of a fluid in the swirl chamber of a nozzle which renders it possible to be able to operate a nozzle with a wide control range and, in the process, to achieve as far as possible a comparable drop quality (mean drop diameter and drop distribution), that is to say to create possibilities of being able to control the mean drop diameter in conjunction with a constant volumetric flow, or to keep the drop spectrum constant in conjunction with controlling the volumetric flow. The aim is also to create a suitable nozzle system for the purpose of carrying out the method.
According to the invention, the object is achieved by means of the features specified in claims 1 and 18. Corresponding variant refinements of the proposed method are specified in claims 2 to 17. Advantageous refinements of the nozzle system are the subject matter of claims 19 to 32.
The proposed method for subdividing the subflows over tangential feed conduits which differ in their cross-sectional surfaces at the connecting point to the swirl chamber, it being the case that upon subdivision of the subflows over more than two tangential feed conduits, the cross-sectional surfaces are formed from the sum of the cross-sectional surfaces of the feed conduits which branch off from the respective subflow, and the sums of the cross-sectional surfaces at the connecting point to the swirl chamber of the respective subflows therefore differ, leads to a substantial widening of the control range during operation of the nozzle systems. The possibility of controlling the drop spectrum in conjunction with a constant volumetric flow, or of keeping the drop spectrum constant in conjunction with variation in the volumetric flow is particularly advantageous in the practical use of the nozzles. The term fluid is to be understood within the scope of the present invention as also including mixtures of different fluids with or without solids. The control possibilities, created by the new method, for different nozzle applications result in improved productivity of the production systems, and in a substantial cost reduction. In order to ensure a wide control range, the cross-sectional surfaces should differ by a factor of more than four. According to the invention, the liquid throughput is subdivided into a plurality of subflows which have different cross-sectional surfaces. It is the cross-sectional surfaces at the inlet of the liquid into the swirl chamber (connecting point of the feed conduit and swirl chamber) which are decisive, since the circumferential speed at the periphery of the swirl chamber is fixed at this point. If the aim is a high swirl intensity for a fine drop spectrum, it is necessary to enlarge the subflow applied to the feed conduits which have the smallest cross section, and vice versa. Intermediate values can be set continuously. The simplest way of influencing the throughput of a subflow is to use a valve. The other object for which the method may be applied is to maintain a specific swirl intensity at the outlet from the swirl chamber. In this case, the ratio of the sum of the cross-sectional surfaces of the feed conduits which are affected in the full load case, and the sum of the cross-sectional surfaces of the feed conduits which are affected in the part load case is to be selected to be at least as high as the desired ratio of the volumetric flows in the cases of full load and part load. The principle of swirl control according to the invention can be applied during atomization of liquids in single-component and double-component nozzles in which either the liquid or the gas or both are provided with a circumferential speed in the nozzle. The application is performed in such a way that the method is applied both [sic] to the liquid or the gas or to both. It is therefore possible to influence the drop quality in the case of double-component nozzles without changing the liquid throughput/gas throughput ratio. The purpose for which the liquid is atomized is not important here. The atomization can be performed, for example, for subsequent drying of a suspension in the dry tower. However, it is also possible to atomize oil which, as customary with burners, is burnt at the nozzle outlet. However, the fluid can also be a gas. This case is possible with multiple-component nozzles, where the gas is provided with a swirl component in order to atomize liquid. However, the gas can also be provided with a swirl component without the presence of liquid, as in the case of gas burners which operate with recirculation in the vicinity of the nozzle outlet. Finally, it is possible to combine the principle according to the invention with the spill-return method, in order to permit a further widening of the control range. With most spray-drying systems, the use of return flow nozzles is precluded for quite different reasons. In the case of these systems, it has previously been necessary to operate with a prescribed nozzle geometry. The frequent changes in the product therefore necessitated a new selection of the nozzle system and, because of the change of nozzle required, the system had to be run up and run down. The new system renders it possible to adapt during operation, and it is even possible to carry out control owing to continuous measurement of the product parameters. Variations in the product parameters which result from wear of the nozzle can be leveled out over a certain time, and the service life of the spray tower can be prolonged thereby. In the case of using the invention in the field of oil combustion, success is achieved in operating with a wide load range without the return line and without varying the jet angle in conjunction with a virtually unchanged drop size. This influences the effectiveness of the entire heating system and the service life of the boiler, since in the case of fluctuating heat requirements there is no need to implement frequent running up and running down of the burner. The method according to the invention can also be successfully applied in the case of gas burners and coal dust burners, chiefly in order to influence the shape of the burner flame. In the case of the application of the invention to fuel atomization in turbines, a reaction to different operating requirements is rendered possible. It is necessary to adapt the fuel atomization in aircraft turbines because of different load requirements (launch period, normal flight) or because of different combustion conditions (the density and composition of air vary as a function of altitude). This is now possible when applying the method according to the invention. Further detailed discussions on the method and the design of the nozzles emerge within the framework of the following exemplary embodiments.